This invention relates to a method for the production of lightly crosslinked polyolefin foams and the foams produced thereby and, more particularly, to a method of producing such foams wherein the crosslinking occurs within the confines of the extrusion equipment and the resulting lightly crosslinked foams have a fine closed cellular structure.
Numerous processes are known for the manufacture of polyolefin foams. Polyolefin foams are classified into two main categories, namely, crosslinked and non-crosslinked polyolefin foams.
Foams in each of the foregoing categories have their application and usage in specific segments of the marketplace. For example, a need for crosslinked foams, notwithstanding their high cost, exists in that market segment which demands that the foam possesses certain mechanical or physical properties, namely, relatively low compression set and compression creep and a high draw rate, and certain thermal properties, namely, relatively high heat resistance.
Non-crosslinked foams, which have a larger cell structure, but poor mechanical and thermal strength, meet a demand which prevails in that market segment which requires lower cost materials, for example, cushioned packaging materials or floatation materials.
The inherent advantages in mechanical properties of crosslinked foams result, in most instances, from their small cell structure, i.e., small cell size, and the fact that closed cells predominate. The advantage in thermal properties of cross-linked foams, such as heat resistance and deep draw, are, in most instances, brought about by the cross-linking of the polyolefin resin itself.
In reviewing the polyolefin foam market, those applications where crosslinked foams are used, instead of non-crosslinked foams, are due mostly to its smaller cell size, which results in its preferred physical and mechanical properties, such as excellent appearance of the foam surface, lower compression set, lower compression creep, deeper draw and good processing during fabrication. While crosslinked foams offer better thermal resistance, purchasers do not regard this feature as being especially significant. Thus, noncrosslinked foams having a small cell size are available to capture a considerable portion of the market for crosslinked foams.
From the standpoint of foam manufacturing processes, there are numerous processes available to manufacture both crosslinked and non-crosslinked foams.
Crosslinked polyolefin foams can be manufactured by any of the following representative processes which are conventional in the art.